US8779323B2 - Electrode for plasma torch with novel assembly method and enhanced heat transfer - Google Patents

Abstract

Embodiments of the present invention are related to an electrode for a plasma arc torch, the electrode comprising a generally tubular outer wall, an end wall, and a protrusion. The end wall is joined to a distal end of the outer wall and supports an emissive element in a generally central region. The protrusion extends from the generally central region of the end wall and is configured to connect with an electrode holder by a releasable connection, wherein the protrusion is configured such that at least one coolant passage forms between the protrusion and the electrode holder when the electrode is connected with the electrode holder. In some embodiments, the releasable connection comprises a threaded connection, wherein the protrusion is threaded to releasably connect to a threaded coolant tube of the electrode holder. In other embodiments, at least one coolant passage is defined by the threaded connection.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation application of pending U.S. patent application Ser. No. 12/957,695, filed Dec. 1, 2010, the entirety of which application is incorporated by reference herein.

FIELD

Embodiments of the present invention relate to electrode assemblies for plasma arc torches and, in particular, to electrodes and electrode holders held to each other or to the plasma arc torch by way of a threaded connection. Further, some embodiments relate to electrode assemblies having defined passages for providing coolant to the electrode.

BACKGROUND

Plasma arc torches are commonly used for the working of metal including cutting, welding, surface treatment, melting and annealing. Such torches include an electrode that supports an arc that extends from the electrode to a workpiece in a transferred-arc mode of operation. To facilitate operation, current is passed to the electrode to create the arc, which heats the electrode to high temperatures, causing erosion and reduction in electrode life. Thus, it is conventional to surround the arc with a vortex flow of plasma gas, and in some torch designs the plasma gas and arc are surrounded by a flow of secondary fluid such as a gas or water.

SUMMARY OF THE INVENTION

In an effort to improve the electrode life and reduce manufacturing costs, embodiments of the present invention provide an electrode for a plasma arc torch with novel assembly method and enhanced heat transfer properties.

One example embodiment is an electrode for a plasma arc torch, the electrode comprising a generally tubular outer wall, an end wall, and a protrusion. The end wall is joined to a distal end of the outer wall and supports an emissive element in a generally central region of the end wall. The protrusion extends from the generally central region of the end wall and is configured to connect with an electrode holder by a releasable connection, wherein the protrusion is configured such that at least one coolant passage forms between the protrusion and the electrode holder when the electrode is connected with the electrode holder. In some embodiments, the releasable connection may comprise a threaded connection and the protrusion may be threaded to releasably connect the protrusion to a threaded coolant tube of the electrode holder. In other embodiments at least one coolant passage may be defined by the threaded connection.

Another embodiment of the present invention is a plasma arc torch comprising a torch body, a nozzle supported adjacent one end of the torch body, an electrode, and an electrode holder. The electrode holder is supported by the torch body and is configured to provide coolant through an interior of the electrode holder. The electrode comprises an end wall that supports an emissive element and a protrusion extending from a generally central region of the end wall. The protrusion connects to the electrode holder by a releasable connection, wherein at least one coolant passage is formed between the protrusion and the electrode holder. The at least one coolant passage allows coolant to flow therethrough and impinge on the end wall of the electrode. In some embodiments, the releasable connection comprises a threaded connection and the protrusion may be threaded to releasably connect to a threaded coolant tube of the electrode holder. In other embodiments, the at least one coolant passage is defined by the threaded connection.

Other embodiments of the present invention include an electrode assembly for a plasma arc torch. The electrode assembly comprising an electrode and an electrode holder. The electrode comprises a generally tubular outer wall, an end wall joined to a distal end of the outer wall and supporting an emissive element in a generally central region of the end wall, and a protrusion extending from the generally central region of the end wall. The electrode holder connects to the electrode by a releasable connection and comprises an inner coolant tube for providing coolant to the electrode and an outer coolant tube surrounding the inner coolant tube for removing coolant from the electrode via a space between the inner and outer coolant tubes. The protrusion of the electrode is configured to connect with the inner coolant tube of the electrode holder by a releasable connection and at least one coolant passage forms between the protrusion of the electrode and the inner coolant tube of the electrode holder when the electrode is connected with the electrode holder. In some embodiments, the releasable connection comprises a threaded connection and the protrusion may be threaded to releasably connect the protrusion to a threaded coolant tube of the electrode holder. In other embodiments, the at least one coolant passage is defined by the threaded connection.

Another embodiment of the present invention is a method for cooling an electrode in a plasma arc torch, comprising the steps of connecting an electrode to an electrode holder by a releasable connection therebetween. The electrode has an end wall supporting an emissive element and a protrusion extending from a generally central region of the end wall, wherein the protrusion is configured to connect with the electrode holder by the releasable connection. The method further comprises providing coolant through a coolant tube of the electrode holder and through at least one coolant passage defined by the releasable connection such that the end wall of the electrode is impinged by the coolant.

BRIEF DESCRIPTION OF THE DRAWINGS

Having thus described the invention in general terms, reference will now be made to the accompanying drawings, which are not necessarily drawn to scale, and wherein:

FIG. 1 is a cross-sectioned side view of a conventional shielding gas plasma arc torch illustrating an electrode assembly as used in the prior art;

FIG. 2 is a cross-sectioned side view of the torch taken along a different section fromFIG. 1 to illustrate coolant flow therethrough;

FIG. 3 is an enlarged cross-sectioned view of the lower portion of the torch as seen inFIG. 1 and illustrating the conventional electrode assembly;

FIG. 4 is an enlarged cross-sectioned view of the lower portion of a plasma arc torch illustrating one example embodiment of an electrode assembly, in accordance with some embodiments discussed herein;

FIG. 5 is a cross-sectioned view of the electrode assembly ofFIG. 4, in accordance with some embodiments discussed herein;

FIG. 6 is an enlarged cross-sectioned view of the lower portion of the electrode assembly ofFIG. 4, in accordance with some embodiments discussed herein;

FIG. 7 is an enlarged cross-sectioned view of an electrode of the electrode assembly ofFIG. 4, in accordance with some embodiments discussed herein;

FIG. 7A is a perspective view of the electrode ofFIG. 7, in accordance with some embodiments discussed herein;

FIG. 8 is an enlarged cross-sectioned view of the lower portion of an electrode holder of the electrode assembly ofFIG. 4, in accordance with some embodiments discussed herein;

FIG. 9 is an enlarged cross-sectioned view of the lower portion of another example embodiment of an electrode assembly, in accordance with some embodiments discussed herein;

FIG. 10 is an enlarged cross-sectioned view of an electrode of the electrode assembly ofFIG. 9, in accordance with some embodiments discussed herein;

FIG. 10A is a perspective view of the electrode ofFIG. 10, in accordance with some embodiments discussed herein;

FIG. 11 is an enlarged cross-sectioned view of the lower portion of an electrode holder of the electrode assembly ofFIG. 9, in accordance with some embodiments discussed herein;

FIG. 12 is an enlarged cross-sectioned view of the lower portion of another embodiment of an electrode assembly, in accordance with some embodiments discussed herein;

FIG. 13 is an enlarged cross-sectioned view of an electrode of the electrode assembly ofFIG. 12, in accordance with some embodiments discussed herein;

FIG. 13A is a perspective view of the electrode ofFIG. 13, in accordance with some embodiments discussed herein;

FIG. 14 is an enlarged cross-sectioned view of an electrode holder of the electrode assembly ofFIG. 12, in accordance with some embodiments discussed herein; and

FIG. 15 is a cross-sectioned view of another embodiment of an electrode assembly, in accordance with some embodiments discussed herein.

DETAILED DESCRIPTION

The present invention now will be described more fully hereinafter with reference to the accompanying drawings, in which some, but not all embodiments of the inventions are shown. Indeed, these inventions may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will satisfy applicable legal requirements. Like numbers refer to like elements throughout.

Plasma arc torches often utilize electrodes that comprise an elongate tubular member composed of a material of high thermal conductivity (e.g., copper, copper alloy, silver, etc.) The forward or discharge end of the tubular electrode includes a bottom end wall having an emissive element embedded therein that supports the arc. The opposite end of the electrode holds the electrode in the torch by way of a releasable connection (e.g., threaded connection) to an electrode holder. The electrode holder is typically an elongate structure held to the torch body by a threaded connection at an end opposite the end at which the electrode is held. The electrode holder and the electrode define a threaded connection for holding the electrode to the electrode holder.

The emissive element of the electrode is composed of a material that has a relatively low work function, which is defined in the art as the potential step, measured in electron volts (eV), which promotes thermionic emission from the surface of a metal at a given temperature. In view of this low work function, the element is thus capable of readily emitting electrons when an electrical potential is applied thereto. Commonly used emissive materials include hafnium, zirconium, tungsten, and alloys thereof.

A nozzle surrounds the discharge end of the electrode and provides a pathway for directing the arc towards the workpiece. To ensure that the arc is emitted through the nozzle and not from the nozzle surface during regular, transferred-arc operation, the electrode and the nozzle are maintained at different electrical potential relative to each other. Thus, it is important that the nozzle and the electrode are electrically separated, and this is typically achieved by maintaining a predetermined physical gap between the components. The volume defining the gap is most typically filled with flowing air or some other gas used in the torch operation.

The heat generated by the plasma arc is great. The torch component that is subjected to the most intense heating is the electrode. To improve the service life of a plasma arc torch, it is generally desirable to maintain the various components of the torch at the lowest possible temperature notwithstanding this heat generation. In some torches, a passageway or bore is formed through the electrode holder, and a coolant such as water is circulated through the passageway to internally cool the electrode.

Even with the water-cooling, the electrode has a limited life span and is considered a consumable part. Thus, in the normal course of operation, a torch operator must periodically replace a consumed electrode by first removing the nozzle and then unthreading the electrode from the electrode holder. A new electrode is then screwed onto the electrode holder and the nozzle is reinstalled so that the plasma arc torch can resume operation.

Other considerations for electrode design include constraints on the threaded connection between the electrode holder and the electrode. For example, the threaded connection must be structurally strong enough to securely hold the electrode to the electrode holder. Additionally, a considerable current is passed through the electrode holder to the electrode, in some cases up to 1,000 amperes of cutting current. Thus, the threaded connection should provide sufficient contact surface area between the electrode and the electrode holder to allow this current to pass through. Finally, the cost of manufacturing the electrode should be as small as possible, especially because the electrode is a frequently replaced consumable part.

Thus, there is a need to increase the useful life of the electrode by more efficient ways to provide coolant, while maintaining low cost of manufacture for the electrode and electrode holder.

The following discussion with respect toFIGS. 1-3 describes a prior plasma arc torch that would benefit from the invention. Aplasma arc torch300 using an electrode and electrode holder according to some embodiments of the present invention is illustrated inFIG. 4. Thus, embodiments of the present invention are described in greater detail with respect toFIGS. 4-15.

FIGS. 1-3 show a priorplasma arc torch10. Thetorch10 is a shielding gas torch, which provides a swirling curtain or jet of shielding gas surrounding the electric arc during a working mode of operation of the torch. Thetorch10 includes a generally cylindrical upper orrear insulator body12 which may be formed of a potting compound or the like, a generally cylindricalmain torch body14 connected to therear insulator body12 and generally made of a conductive material such as metal, a generally cylindrical lower orfront insulator body16 connected to themain torch body14, anelectrode holder assembly18 extending through themain torch body14 andfront insulator body16 and supporting anelectrode20 at a free end of the electrode holder assembly, and anozzle assembly22 connected to theinsulator body16 adjacent theelectrode20.

A plasmagas connector tube24 extends through therear insulator body12 and is connected by screw threads (not shown) into aplasma gas passage26 of themain torch body14. Theplasma gas passage26 extends through themain torch body14 to alower end face28 thereof for supplying a plasma gas (sometimes referred to as a cutting gas), such as oxygen, air, nitrogen, or argon, to a corresponding passage in theinsulator body16.

A shieldinggas connector tube30 extends through therear insulator body12 and is connected by screw threads into a shieldinggas passage32 of themain torch body14. The shieldinggas passage32 extends through themain torch body14 to thelower end face28 for supplying a shielding gas, such as argon or air, to a corresponding passage in theinsulator body16.

Theinsulator body16 has an upper end face34 that abuts thelower end face28 of the main torch body. Aplasma gas passage36 extends through theinsulator body16 from the upper end face34 into acylindrical counterbore38 in the lower end of theinsulator body16. As further described below, thecounterbore38, together with the upper end of thenozzle assembly22, forms aplasma gas chamber40 from which plasma gas is supplied to a primary or plasma gas nozzle of the torch. As such, plasma gas from a suitable source enters theplasma gas chamber40 by flowing through the plasmagas connector tube24, through theplasma gas passage26 in themain torch body14, into theplasma gas passage36 of theinsulator body16, which is aligned with thepassage26, and into thechamber40.

The nozzle, which is illustrated as a two-part nozzle assembly22, includes anupper nozzle member42, which has a generally cylindrical upper portion slidingly received within ametal insert sleeve44 that is inserted into thecounterbore38 of theinsulator body16. An O-ring46 seals the sliding interconnection between theupper nozzle member42 and themetal insert sleeve44. Alower nozzle tip48 of generally frustoconical form also forms a part of thenozzle assembly22, and is threaded into theupper nozzle member42. Thelower nozzle tip48 includes anozzle exit orifice50 at the tip end thereof. Thelower nozzle tip48 andupper nozzle member42 could alternatively be formed as one unitary nozzle. In either configuration, the nozzle channels the plasma gas from a largerdistal opening49 to theexit orifice50. A plasma gas flow path thus exists from theplasma gas chamber40 through thenozzle chamber41 for directing a jet of plasma gas out thenozzle exit orifice50 to aid in performing a work operation on a workpiece.

The plasma gas jet preferably has a swirl component created, in a known manner, by a hollow cylindricalceramic gas baffle52 partially disposed in acounterbore recess54 of theinsulator body16. A lower end of thebaffle52 abuts an annular flange face of theupper nozzle member42. Thebaffle52 has non-radial holes (not shown) for directing plasma gas from theplasma gas chamber40 into a lower portion of thenozzle chamber41 with a swirl component of velocity.

Theelectrode holder assembly18 includes atubular electrode holder56 which has its upper end connected bythreads11 within a blindaxial bore58 in themain torch body14. Theelectrode holder56 is somewhat consumable, although usually less so than the electrode itself, and thus the electrode holder and theaxial bore58 can also be provided with a threaded connection according to the present invention as discussed below. The upper end ofelectrode holder56 extends through anaxial bore60 formed through theinsulator body16, and the lower end of theelectrode holder56 includes an enlarged internally screw-threadedcoupler62 which has an outer diameter slightly smaller than the inner diameter of theceramic gas baffle52 which is sleeved over the outside of thecoupler62. Theelectrode holder56 also includes internal screw threads spaced above thecoupler62 for threadingly receiving acoolant tube64 which supplies coolant to theelectrode20, as further described below, and which extends outward from the axial bore of theinsulator body16 into the central passage of theelectrode20. To prevent improper disassembly or reassembly of thecoolant tube64 and theelectrode holder56, the screw thread connection between those items may be cemented or otherwise secured together during manufacture to form an inseparableelectrode holder assembly18. Theelectrode20 may be of the type described in U.S. Pat. No. 5,097,111, assigned to the assignee of the present application, and incorporated herein by reference.

Theprior art electrode20 comprises a cup-shaped body whose open upper end is threaded byscrew threads63 into thecoupler62 at the lower end of theelectrode holder56, and whose capped lower end is closely adjacent the lower end of thecoolant tube64. A coolant circulating space exists between the inner surface of the wall of theelectrode20 and the outer surface of the wall of thecoolant tube64, and between the outer surface of the wall of thecoolant tube64 and the inner surface of the wall of theelectrode holder56. Theelectrode holder56 includes a plurality ofholes66 for supplying coolant from the space within the electrode holder to aspace68 between the electrode holder and the inner wall of theaxial bore60 in theinsulator body16. Aseal69 located between theholes66 and thecoupler62 seals against the inner wall of thebore60 to prevent coolant in thespace68 from flowing past theseal69 toward thecoupler62. A raised annular rib ordam71 on the outer surface of theelectrode holder56 is located on the other side of theholes66 from theseal69, for reasons which will be made apparent below. A coolant supply passage70 (FIG. 2) extends through the insulator body from thespace68 through the outer cylindrical surface of theinsulator body16 for supplying coolant to thenozzle assembly22, as further described below.

During starting of thetorch10, a difference in electrical voltage potential is established between theelectrode20 and thenozzle tip48 so that an electric arc forms across the gap therebetween. Plasma gas is then flowed through thenozzle assembly22 and the electric arc is blown outward from thenozzle exit orifice50 until it attaches to a workpiece, at which point thenozzle assembly22 is disconnected from the electric source so that the arc exists between theelectrode20 and the workpiece. The torch is then in a working mode of operation.

For controlling the work operation being performed, it is known to use a control fluid such as a shielding gas to surround the arc with a swirling curtain of gas. To this end, theinsulator body16 includes a shieldinggas passage72 that extends from the upper end face34 axially into the insulator body, and then angles outwardly and extends through the cylindrical outer surface of the insulator body. A nozzle retainingcup assembly74 surrounds theinsulator body16 to create a generally annular shieldinggas chamber76 between theinsulator body16 and the nozzle retainingcup assembly74. Shielding gas is supplied through the shieldinggas passage72 of theinsulator body16 into the shieldinggas chamber76.

The nozzle retainingcup assembly74 includes a nozzle retainingcup holder78 and anozzle retaining cup80 which is secured within theholder78 by asnap ring81 or the like. The nozzle retainingcup holder78 is a generally cylindrical sleeve, preferably formed of metal, which is threaded over the lower end of a torchouter housing82 which surrounds themain torch body14.Insulation84 is interposed between theouter housing82 and themain torch body14. Thenozzle retaining cup80 preferably is formed of plastic and has a generally cylindrical upper portion that is secured within thecup holder78 by thesnap ring81 and a generally frustoconical lower portion which extends toward the end of the torch and includes an inwardly directedflange86. Theflange86 confronts an outwardly directedflange88 on theupper nozzle member42 and contacts an O-ring90 disposed therebetween. Thus, in threading the nozzle retainingcup assembly74 onto theouter housing82, thenozzle retaining cup80 draws thenozzle assembly22 upward into themetal insert sleeve44 in theinsulator body16. Thenozzle assembly22 is thereby made to contact an electrical contact ring secured within thecounterbore38 of theinsulator body16. More details of the electrical connections within the torch can be found in commonly-owned U.S. Pat. No. 6,215,090, which is incorporated by reference herein in its entirety.

Thenozzle retaining cup80 fits loosely within thecup holder78, and includeslongitudinal grooves92 in its outer surface for the passage of shielding gas from thechamber76 toward the end of the torch. Alternatively or additionally, grooves (not shown) may be formed in the inner surface of thecup holder78. A shieldinggas cup94 of generally frustoconical form concentrically surrounds and is spaced outwardly of thelower nozzle tip48 and is held by ashield retainer96 that is threaded over the lower end of thecup holder78. A shieldinggas flow path98 thus extends from thelongitudinal grooves92 in retainingcup80, between theshield retainer96 and the retainingcup80 andupper nozzle member42, and between the shieldinggas cup94 and thelower nozzle tip48.

The shieldinggas cup94 includes adiffuser100 that in known manner imparts a swirl to the shielding gas flowing into the flow path between the shieldinggas cup94 and thelower nozzle tip48. Thus, a swirling curtain of shielding gas is created surrounding the jet of plasma gas and the arc emanating from thenozzle exit orifice50.

With primary reference toFIG. 2, the coolant circuits for cooling theelectrode20 andnozzle assembly22 are now described. Thetorch10 includes a coolantinlet connector tube112 that extends through therear insulator body12 and is secured within acoolant inlet passage114 in themain torch body14. Thecoolant inlet passage114 connects to the center axial bore58 in the main torch body. Coolant is thus supplied into thebore58 and thence into the internal passage through theelectrode holder56, through the internal passage of thecoolant tube64, and into the space between thetube64 and theelectrode20. Heat is transferred to the liquid coolant (typically water or antifreeze) from the lower end of the electrode (from which the arc emanates) and the liquid then flows through a passage between the lower end of thecoolant tube64 and theelectrode20 and upwardly through the annular space between thecoolant tube64 and theelectrode20, and then into the annular space between thecoolant tube64 and theelectrode holder18.

The coolant then flows out through theholes66 into thespace68 and into thepassage70 through theinsulator body16. Theseal69 prevents the coolant in thespace68 from flowing toward thecoupler62 at the lower end of theholder56, and thedam71 substantially prevents coolant from flowing past thedam71 in the other direction, although there is not a positive seal between thedam71 and the inner wall of thebore60. Thus, the coolant inspace68 is largely constrained to flow into thepassage70. Theinsulator body16 includes a groove or flattenedportion116 that permits coolant to flow from thepassage70 between theinsulator body16 and thenozzle retaining cup80 and into acoolant chamber118 which surrounds theupper nozzle member42. The coolant flows around theupper nozzle member42 to cool the nozzle assembly.

Coolant is returned from the nozzle assembly via a second groove or flattenedportion120 angularly displaced from theportion116, and into acoolant return passage122 in theinsulator body16. Thecoolant return passage122 extends into a portion of theaxial bore60 that is separated from thecoolant supply passage70 by thedam71. The coolant then flows between theelectrode holder56 and the inner wall of thebore60 and thebore58 in themain torch body14 into anannular space126 which is connected with acoolant return passage128 formed in themain torch body14, and out thecoolant return passage128 via a coolantreturn connector tube130 secured therein. Typically, returned coolant is recirculated in a closed loop back to the torch after being cooled.

In use, and with reference toFIG. 1, one side of an electricalpotential source210, typically the cathode side, is connected to themain torch body12 and thus is connected electrically with theelectrode20, and the other side, typically the anode side, of thesource210 is connected to thenozzle assembly22 through aswitch212 and aresistor214. The anode side is also connected in parallel to theworkpiece216 with no resistor interposed therebetween. A high voltage and high frequency are imposed across the electrode and nozzle assembly, causing an electric arc to be established across a gap therebetween adjacent the plasma gas nozzle discharge. Plasma gas is flowed through the nozzle assembly to blow the pilot arc outward through the nozzle discharge until the arc attaches to the workpiece. Theswitch212 connecting the potential source to the nozzle assembly is then opened, and the torch is in the transferred arc mode for performing a work operation on the workpiece. The power supplied to the torch is increased in the transferred arc mode to create a cutting arc, which is of a higher current than the pilot arc.

Example embodiments of the present invention are illustrated inFIGS. 4-15.FIG. 4 shows one example embodiment of aplasma arc torch300 comprising anelectrode holder assembly318 and novel threaded connection. Theelectrode holder assembly318 comprises anelectrode holder356 and anelectrode320. Although illustrated herein with a torch that uses a high-frequency pilot signal to start an arc, the electrode and electrode holder according to the invention can also be used with blowback-type torches.

With reference toFIGS. 4-6, theelectrode holder356 is tubular and comprises an upper end connected bythreads11 within the blind axial bore in the main torch body, as described above, and a lower end connected to theelectrode320. Theelectrode holder356 comprises aninner coolant tube352 and anouter coolant tube354. Theinner coolant tube352 supplies coolant to theelectrode320. Theouter coolant tube354 is generally tubular shaped and annularly surrounds theinner coolant tube352. Theouter coolant tube354 is configured to remove coolant from theelectrode320. Theelectrode holder356 can be formed from a variety of different electrically conductive materials, but in one embodiment theelectrode holder356 is made of brass or a brass alloy.

Theelectrode320, shown in cross-sectional view inFIG. 7 and perspective view inFIG. 7A, is generally cup-shaped. In the depicted embodiment, theelectrode320 comprises anouter wall335, anend wall330, and aprotrusion325. Theouter wall335 is generally tubular shaped and, with reference toFIG. 6, may be configured to engage with theelectrode holder356 such that anouter passage337 is formed between the exterior surface of theinner coolant tube352 and the interior surface of theouter wall335, allowing coolant to pass through theouter passage337 to theouter coolant tube354. Theend wall330 joins to a distal end of theouter wall335 and supports anemissive element332 in a generally central region of theend wall330. Theprotrusion325 extends from the generally central region of theend wall330 and is configured to connect with theelectrode holder356 by a releasable connection, such as a threaded connection shown in the depicted embodiment. Theelectrode320 can be formed from a variety of different electrically conductive materials, but in one embodiment theelectrode320 comprises a body made of copper or a copper alloy.

With reference toFIGS. 6 and 7,threads310 secure theelectrode320 to theelectrode holder356. In the depicted embodiment, theinner coolant tube352 of theelectrode holder356 has a female threadedportion317 formed therein on the interior surface of theinner coolant tube352. Theprotrusion325 of theelectrode320 has a male threadedportion319 formed thereon on the exterior surface of theprotrusion325. The female threadedportion317, shown inFIG. 8, may be formed on the lower end of theinner coolant tube352 and configured to releasably receive the male threadedportion319 of theprotrusion325. The male threadedportion319 may comprisethreads310 configured as any type of thread, such as a double start screw thread, a metric screw thread, a unified screw thread, a British standard pipe thread, a Whitworth screw thread, or a screw thread having a stub acme profile as described in U.S. Pat. No. 7,081,597, assigned to assignee of the present invention, and which is hereby incorporated herein by reference. The female threadedportion317 may also be configured with a thread profile to match a male threadedportion319 withthreads310 configured as any type of thread, such as those screw threads listed above.

In some embodiments, theprotrusion325 may be further configured such that at least onecoolant passage360 forms between theprotrusion325 and theelectrode holder356 when theelectrode320 is connected to theelectrode holder356. In the depicted embodiment, the male threadedportion319 and the female threadedportion317 are configured with extra space between thethreads310 so that coolant can flow between thethreads310. In particular, as shown inFIG. 6, the extra space can form at least onecoolant passage360 between theprotrusion325 and theinner coolant tube352 of theelectrode holder356. In the depicted embodiment, thecoolant passage360 comprises a helically extending space between the thread profile of the male threadedportion319 on theprotrusion325 and the thread profile of the female threadedportion317 on theinner coolant tube352. Thus, coolant enters thecoolant passage360 from theinner coolant tube352, spirals around thecoolant passage360 within the threads, and exits thecoolant passage360 to impinge theend wall330. The coolant then flows into theouter passage337 and away from theelectrode320 through theouter coolant tube354. Thesenovel coolant passages360, which follow the threaded connection of theelectrode assembly318, allow the flowing coolant to contact inner surfaces of the electrode for a greater amount of time relative to an electrode such as shown inFIGS. 1-3. Additionally, higher coolant velocity may be achieved, which will improve convective heat transfer from the electrode to the coolant. The net result should be enhanced cooling of theelectrode320, which in turn should prolong the life of theelectrode320.

In the depicted embodiment, theinner coolant tube352 of theelectrode holder356 comprises abottom end353 extending generally toward theelectrode320. Different configurations of theelectrode assembly318 may require thebottom end353 of theinner coolant tube352 to be positioned properly with respect to theelectrode320 for the plasma arc torch to function properly. For example, in some embodiments, anopening363 may be configured to allow coolant to flow from thecoolant passage360 to theouter passage337 formed between theinner coolant tube352 and theouter wall335 of theelectrode320, thereby allowing the coolant to be ultimately removed from theelectrode320 through theouter coolant tube354. In the depicted embodiment, theopening363 is configured as additional space between thebottom end353 of theinner coolant tube352 and theend wall330 of theelectrode320. In some embodiments, this may be accomplished by configuring the female threadedportion317 to extend only partially up the lower end of theinner coolant tube352 such that theprotrusion325 can be screwed into the female threadedportion317 for only a certain distance, ensuring theopening363 to form. Alternatively or additionally, a stopper (not shown) can be positioned within the female threadedportion317 to prevent the male threadedportion319 of theprotrusion325 from being advanced past a certain point (i.e., the position of the stopper), thereby ensuring theopening363 forms.

Alternatively or additionally, in other embodiments, theopening363 may comprise a slot (not shown) in theinner coolant tube352 that connects thecoolant passage360 to theouter passage337. The slot may be configured adjacent to the bottom of thecoolant passage360 such that coolant flows through theentire coolant passage360, out of the slot, into theouter passage337, and up through theouter coolant tube354. In some embodiments, the slot can allow the coolant to flow through theelectrode assembly318 with greater velocity, improving the convective heat transfer and prolonging the life of theelectrode320.

FIGS. 9-11 show another embodiment of the present invention, wherein anelectrode assembly418 utilizes a double start screw thread for the releasable connection between theelectrode420 and theelectrode holder456. However, theelectrode assembly418 may be used in a plasma arc torch in similar manner as theelectrode assembly318 described above with respectFIGS. 4-8, as well as with other embodiments of the present invention as described herein.

In the depicted embodiment ofFIG. 9, theelectrode assembly418 comprises anelectrode420 and anelectrode holder456. Theelectrode420 comprises aprotrusion425 with a male threadedportion419 defined bythreads410 that form a double-start screw thread. Theelectrode holder456 comprises aninner coolant tube452 with a female threadedportion417 configured with a thread profile that matches the double start screw thread of the male threadedportion419 to allow for a releasable connection. Furthermore, the thread profile of the male and female threadedportions419,417, similar to the male and female threadedportions319,317 of theelectrode assembly318, may also form at least onecoolant passage460. In the depicted embodiment, theelectrode assembly418 comprises twocoolant passages460 and460′, formed separately due to the thread profile of the double start screw thread. As such, in some embodiments, coolant may flow through bothcoolant passages460,460′ out ofrespective openings463,463′ and away from theelectrode420 through theouter coolant passage454. In the depicted embodiment, theopenings463,463′ comprise slots formed in theinner coolant tube452.

FIGS. 12-14 show another embodiment of the present invention, wherein theelectrode assembly518 comprises anelectrode520 and anelectrode holder556 having a threaded connection between an exterior surface of aninner coolant tube552 and an interior surface of aprotrusion525. Theelectrode assembly518 may also be used in a plasma arc torch in similar manner as theelectrode assembly318 described with respect toFIGS. 4-8, as well as with other embodiments of the present invention as described herein.

With reference toFIG. 12, theelectrode520 comprises an annularly shapedprotrusion525 having a female threadedportion517. Theelectrode holder556 comprises aninner coolant tube552 having a male threadedportion519 comprisingthreads510. The thread profile of the male threadedportion519 and the corresponding female threadedportion517 may be any type of screw thread, such as a double start screw thread, a metric screw thread, a unified screw thread, a British standard pipe thread, a Whitworth screw thread, or a screw thread having a stub acme profile.

In the depicted embodiment, theinner coolant tube552 and the central bore of theprotrusion525 define aninner passage536 configured to allow coolant to flow to theelectrode520. In some embodiments, theinner passage536 defines a reservoir positioned directly above anend wall530 and anemissive element532 contained within theend wall530. Thus, coolant in the reservoir will directly contact the portion of theelectrode520 with the highest temperature (i.e., the portion near the emissive element532).

Additionally, in the depicted embodiment, theelectrode520 further comprises at least one passage orslot576. One end of theslot576 connects theinner passage536 to anouter passage537 defined between theprotrusion525 and theouter wall535. Therefore, theslot576 allows coolant to flow from theinner passage536 to theouter passage537, so that the coolant can ultimately flow away from the electrode through the outer coolant tube554 of theelectrode holder556.

FIG. 15 shows another embodiment of anelectrode assembly618 with anelectrode holder656 and an attachedelectrode620. In the depicted embodiment, theelectrode620 comprises a tubularouter wall635 that extends past theprotrusion625. Coolant can flow through theinner coolant tube652 of theelectrode holder656 and around theprotrusion625 to cool theelectrode620, as described in various embodiments above. Furthermore, the coolant can flow between the exterior surface of theinner coolant tube652 and the interior surface of theouter wall635 to theouter coolant tube654 for removal from theelectrode620.

Another embodiment of the present invention includes a method for cooling an electrode in a plasma arc torch comprising providing coolant to the electrode through various embodiments of the invention as described herein. In particular, the method may comprise the steps of connecting an electrode to an electrode holder by a releasable connection therebetween, the electrode having an end wall supporting an emissive element and a protrusion extending from a generally central region of the end wall, the protrusion being configured to connect with the electrode holder by the releasable connection. The method may further comprise providing coolant through a coolant tube of the electrode holder and through at least one coolant passage defined by the releasable connection such that the end wall of the electrode is impinged by the coolant. In some embodiments, the method may further comprise removing coolant from the at least one coolant passage through at least one slot adjacent to the coolant passage. The method may also further comprise removing coolant from the electrode through an outer coolant tube defined in the electrode holder. In other embodiments, the step of providing coolant through the at least one coolant passage comprises passing coolant through a helically extending space between a thread profile on the protrusion of the electrode body and a thread profile on the coolant tube of the electrode holder.

Embodiments of the present invention as described herein address releasable connections and the issue of heat transfer for electrode cooling methods. In particular, some embodiments utilize cooling passages formed between the thread profile of the electrode and the inner coolant tube of the electrode holder to increase coolant flow velocity and increase the surface area used for heat transfer between the electrode and the coolant. Other embodiments described herein also advantageously utilize slots to facilitate coolant flow. In fact, the combination of the coolant passages and the slots has shown to increase flow velocity by three fold. Thus, embodiments of the present invention improve heat transfer by increasing flow velocity and increasing the surface area of the electrode that interacts with the coolant, thereby increasing the useable life of the electrode in a plasma arc torch.

Many modifications and other embodiments of the inventions set forth herein will come to mind to one skilled in the art to which these inventions pertain having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is to be understood that the inventions are not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included herein. Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.

Claims (20)

What is claimed is:

1. An electrode for a plasma arc torch, the electrode comprising:

a generally tubular outer wall;

an end wall joined to a distal end of the outer wall and supporting an emissive element in a generally central region of the end wall;

a protrusion extending from the generally central region of the end wall, the protrusion centrally disposed within the generally tubular outer wall, the protrusion comprising threads for securing the electrode to threads of an electrode holder; and

a coolant passage formed between the protrusion and the electrode holder when the electrode is connected with the electrode holder.

2. The electrode ofclaim 1, wherein the protrusion threads comprise female threads for securing the electrode to male threads of the electrode holder.

3. The electrode ofclaim 1, wherein the coolant passage comprises a slot.

4. The electrode ofclaim 3, wherein the slot comprises a radially disposed opening coupling an inner passage of the protrusion to an outer passage of the protrusion.

5. The electrode ofclaim 4, wherein the slot comprises a plurality of radially disposed openings coupling the inner passage of the protrusion to an outer passage of the protrusion, the plurality of radially disposed openings positioned in spaced apart relation about a circumference of the protrusion.

6. The electrode ofclaim 1, wherein at least a portion of the coolant passage is defined by the threaded connection between the electrode and the electrode holder.

7. The electrode ofclaim 1, wherein the threaded connection is configured as one of a double start screw thread, a metric screw thread, a unified screw thread, a British standard pipe thread, a Whitworth screw thread, and a screw thread having a stub acme profile.

8. The electrode ofclaim 1, wherein the protrusion is further configured such that a gap forms between the end wall and the coolant tube of the electrode holder when the electrode is connected to the electrode holder.

9. The electrode ofclaim 1, wherein the protrusion is annular and comprises an interior surface, and wherein at least a portion of the interior surface of the protrusion is configured to connect with the coolant tube of the electrode holder by the threaded connection.

10. An electrode assembly for a plasma arc torch, comprising:

an electrode comprising; a generally tubular outer wall;

an end wall joined to a distal end of the outer wall and supporting an emissive element in a generally central region of the end wall; and

a protrusion extending from the generally central region of the end wall, at least a portion of the protrusion being centrally disposed within the generally tubular outer wall of the electrode; and

an electrode holder connected to the electrode, the electrode holder comprising:

an inner coolant tube for providing coolant to the electrode; and

an outer coolant tube surrounding the inner coolant tube for removing coolant from the electrode via a space between the inner and outer coolant tubes;

wherein the protrusion of the electrode includes threads for connecting with threads of the inner coolant tube of the electrode holder,

wherein a coolant passage is formed between the protrusion and the electrode holder when the electrode is connected with the electrode holder.

11. The electrode assembly ofclaim 10, wherein the protrusion threads comprise female threads for securing the electrode to male threads of the electrode holder.

12. The electrode assembly ofclaim 10, wherein the coolant passage comprises a slot.

13. The electrode assembly ofclaim 12, wherein the slot comprises a radially disposed opening coupling an inner passage of the protrusion to an outer passage of the protrusion.

14. The electrode assembly ofclaim 13, wherein the slot comprises a plurality of radially disposed openings coupling the inner passage of the protrusion to an outer passage of the protrusion, the plurality of radially disposed openings positioned in spaced apart relation about a circumference of the protrusion.

15. The electrode assembly ofclaim 10, wherein the protrusion is annular and comprises an interior surface, and wherein at least a portion of the interior surface of the protrusion is configured to connect with the coolant tube of the electrode holder by the threaded connection.

16. The electrode assembly ofclaim 10, wherein the protrusion is further configured such that a gap forms between the end wall and the coolant tube of the electrode holder when the electrode is connected to the electrode holder.

17. A method for cooling an electrode in a plasma arc torch, comprising the steps of:

connecting an electrode to an electrode holder by a threaded connection therebetween, the electrode having an end wall supporting an emissive element and a protrusion extending from a generally central region of the end wall, the protrusion centrally disposed within a generally tubular outer wall of the electrode, the protrusion including threads for securing the electrode to threads of the electrode holder; and

providing coolant through a coolant tube of the electrode holder and through at least one coolant passage defined between the protrusion and the electrode holder such that the end wall of the electrode is impinged by the coolant.

18. The method ofclaim 17, wherein the coolant passage comprises a slot, the method comprising moving coolant through the slot.

19. The method ofclaim 17, further comprising removing coolant from the electrode through an outer coolant tube defined in the electrode holder.

20. The method ofclaim 17, wherein the coolant passage comprises a plurality of radially disposed slots, the method comprising moving coolant through the plurality of slots such that the end wall of the electrode is impinged by the coolant.

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